1. Field of the Invention
The present invention relates to an injection molding machine capable of providing feedback control of an injection pressure for making it equal to a target injection pressure in the injection and dwell stages, more particularly, to a method for setting waveform of an injection pressure to be used in setting a target value and an injection molding machine for carrying out this method.
2. Description of the Related Art
Generally, according to a conventional injection molding machine, the injection speed is set in accordance with the advanced position of a screw, and the advancing speed of the screw is controlled to be made equal to the set injection speed, in an injection stage. In a dwell stage, the machine is controlled so that a set dwell pressure is applied to a resin.
In an actual molding operation, however, the propriety of the injection pressure influences the conformity of molded articles much more greatly than that of the injection speed. It is desirable, therefore, that the injection pressure should be feedback-controlled during the injection and dwell stages. The inventor hereof proposed a control system in which a pressure sensor is mounted on a screw shaft, and the dwell pressure is feedback-controlled by detecting a pressure of the resin acting on the screw shaft. This is disclosed in Published Unexamined Japanese Patent Application No. 62-218118. However, the system disclosed in this published application is associated with pressure control for the dwell stage only.
Disclosed in Published Examined Japanese Patent Application No. 58-52486, is a hydraulically-operated injection molding machine in which the in-mold pressure is detected by means of a pressure sensor provided in a resin passage of a mold, and is feedback-controlled to be made equal to a set pressure. In this case, the in-mold pressure is detected, so that the pressure of the resin during injection cannot be detected. Thus, the pressure acting on the resin cannot be accurately detected.
Thus, the inventor developed an injection molding machine in which a pressure sensor is mounted on a screw shaft so that not only the pressure applied to a resin during an injection/dwell stage can be detected but also the change of the pressure acting on the resin during the injection/dwell stage is optionally set as an injection pressure waveform based on a function of time, and the injection pressure is feedback-controlled so that an actual injection pressure waveform detected by means of the pressure sensor coincides with the set injection pressure waveform, and the inventor has filed an application for a patent in Japan. This application is published as Published Unexamined Japanese Patent Application No. 3-58821.
In this injection molding machine for feedback-controlling injection/dwell pressure, described in Published Unexamined Japanese Patent Application No. 3-58821, the injection pressure waveform as a target value can be set optionally. It is impossible, however, to set an existing injection pressure waveform or a modified injection pressure waveform obtained by modifying the existing injection pressure waveform as a base. Thus, the injection pressure waveform must always be set from the beginning.
Some of similar molds are designed so that an injection pressure waveform suited for one mold can be obtained by partially modifying an injection pressure waveform set for another. The injection pressure waveform can be set very easily if the injection pressure waveform of the one mold can be set by referring to and partially modifying the injection pressure waveform set for the other mold. If no conforming molded article can be obtained by trial injection executed after the feedback control of the injection/dwell pressure based on the once set injection pressure waveform, the injection pressure waveform should be set again. Setting the injection pressure waveform again from the beginning in such a case, is a waste of time and labor, and so it is desirable to modify the previously set injection pressure waveform so that the modified injection pressure waveform can be set.
Further, when the trial injection is executed after the injection pressure waveform is set, the set injection pressure waveform may undergo a sudden change such that the injection molding machine cannot follow the change, thus entailing a substantial difference between the actual injection pressure waveform and the set injection pressure waveform, or a conforming molded article may not be able to be obtained. In such a case, setting of the injection pressure waveform can be facilitated by modifying part of the actual injection pressure waveform with reference to this actual injection pressure waveform so that the modified version can be set as the injection pressure waveform. Also, it is desirable that the actual injection pressure waveform observed when the conforming molded article is obtained should be able to be set directly as the set injection pressure waveform.